Atomic hydrogen maser with enlarged atom storage container



June 11, 1968 N. F. RAMSEY 3,388,342

ATOMIC HYDROGEN MASER WITH ENLARGED ATOM STORAGE CONTAINER Filed March29, 1967 i 2 3 ATOMIC J l HYDROGEN D i AMPLIFIER SOURCE 1 N VEN TOR.

Norman F. Ramsey United States Patent 3,388,342 ATOMIC HYDROGEN MASERWITH ENLARGED ATOM STORAGE CONTAINER Norman F. Ramsey, Belmont, Mass.,assignor to the United States of America as represented by the Secretaryof the Navy Filed Mar. 29, 1967, Ser. No. 627,593 5 Claims. (Cl. 331-94)ABSTRACT OF THE DISCLOSURE An atomic hydrogen maser including an atomicstorage container which is made large compared to the wavelength of thehyperfine transition frequency of hydrogen to reduce undesirable walleffects. Two smaller cavity resonators which may contain atom storagebulbs are connected with the interior of the storage container, and RF.energy from one of the resonators is amplified to produce an outputsignal a portion of which is applied to excite the other resonator.

The invention described herein may be manufactured and used by or forthe Government of the United States of America for governmental purposeswithout the payment of any royalties thereon or therefor.

In the hydrogen maser, atomic hydrogen from a RF. discharge score passesthrough the inhomogeneous stateselecting magnetic field of a six-polepermanent magnet. This field focuses atoms in the [F =1, m=0] and [F=1,m=l] state onto an aperture in a specially coated dielectric bulb. Thisbulb is located in a region of constantphase axial R.F. magnetic fieldat the center of a cylindrical cavity resonator excited in the 'IE modeand tuned to the [F: 1, 112:1] [F=0, 112:0] hyperfine transitionfrequency of approximately 1420.405 mc./sec. The atoms which enter thisbulb make random collisions with the coated bulb wall and eventuallyleave the bulb through the entrance aperture. Due to their smallinteraction with the wall coating, which has a particularcharacteristic, the atoms are not seriously perturbed even though theyare retained in the bulb for more than a second and undergo more thancollisions with the Wall during this period.

Since the transition time due to the storage feature just mentioned islonger than one second, the resonant line of the radiated energy isextremely narrow. Thus, self-excited maser oscillation at the hyperfinefrequency may take place, and this oscillation occurs whenever theenergy delivered to the R.F. cavity by the beam equals that lost by thisdevice.

The principal source of error in absolute time and frequencymeasurements with an atomic hydrogen maser of the type just describedarises from the effect of the wall of the storage bulb on the frequencyof the maser. Even though this effect is small and may be measured byexperiments using storage bulbs of two different diameters, it does,nevertheless,- contribute a slight error. One method of minimizing thiswould be to resort to new wall materials which would permit the atoms toexperience an even further number of wall collisions and still notundergo transitions between hyperfine energy states or any other form ofrelaxation which would lead to a loss of coherence.

The wall effects may be reduced, of course, by going to a largerdiameter storage bulb, for the fraction of time the atom spends on thewall is inversely proportional to the storage bulb diameter. In thepast, it has been necessary to limit the storage bulb to a smalldiameter, approximately six inches, so that it may be confined to aregion of a single oscillatory phase in the RF. cavity.

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The dimensions of this cavity, of course, are invariably fixed by thewavelength of the hydrogen hyperfine radiation. This limitation isovercome in the present invention because the main storage container iseffectively external to the excited R.F. cavity. The size of thiscontainer, therefore, need not be limited by the operating frequency ofthe atomic hydrogen maser. For example, in one proposed embodiment ofthe present invention, the storage container has a diameter ofapproximately sixty inches, and this decreases the wall effects by afactor of ten. It also increases the storage time by a similar factor.Thus, the system achieves an increase in line Q and a correspondingreduction in frequency drift due to instability of the tuned cavity.

The present invention is not only superior for absolute time andfrequency measurements, but it also provides greater accuracy in themeasurement of other hyperfine separations, such as, deuterium andtritium. Moreover, the present limitation in the accuracy of spin,exchange, cross-section measurements with available hydrogen masers,which is due to pressure-dependent wall effects, is reduced with thelarger storage box available in the present system.

It is accordingly a primary object of the present invention to providean atomic hydrogen maser having improved frequency accuracy andstability.

Another object of the present invention is to provide an atomic hydrogenmaser having an atomic storage box 7 whose size is unlimited by thewavelength of the hyperfine transition frequency.

Other objects, advantages and novel features of the invention willbecome apparent from the following detailed description of the inventionwhen considered in conjunction with the accompanying drawings wherein:

FIG. 1 schematically depicts one embodiment of the present invention;and

FIG. 2 illustrates an alternative arrangement with the high level tunedcavity in a different location in the system.

Referring now to FIG. 1, it will be seen that an atomic hydrogen source1 and a six-pole, state-selecting magnet 2, both of which may be similarin construction to the apparatus disclosed in applicants U.S. Patent No.3,255,- 423 of Jan. 7, 1966, cooperate to focus atoms in the higher F :1hyperfine states through the defining aperture 3 of a mask 4- and into acylindrical storage container 5. This container is constructed with anexterior entrance passageway 6 which acts as a collimator to permit onlythe axially directed atoms in the beam to enter into its interior. Sincecontainer 5, unlike prior art devices, is not Within the excited R.F.cavity, this unit need not be made of dielectric material. For the samereason, its size may be much larger than the wavelength of the hyperfinetransition frequency.

Mounted on an exterior upper and lower wall portion of container 5 are apair of RF. cavity resonators 7 and 8. These cavity resonators aresimilar to those usually employed in the conventional atomic hydrogenmaser. That is, both cavities operate in the TE mode and are tuned tothe hyperfine transition frequency of 1420.40 mc. Accommodated withineach cavity is a dielectric storage bulb 9 and 10. These bulbs are alsothe equivalent of those used in the prior art hydrogen maser. Their sizeis determined by the restriction mentioned hereinbefore that they beconfined to a region of a single oscillatory phase of the RF. energy inthe resonant cavities. Consequently, they may be approximately sixinches in diameter.

Both bulbs are in communication with the interior of the main storagebox 5. Thus, any atoms within this container may enter these smallerbulbs, remain stored therein for a short period of time and interactwith any of the RF. energy present therein. Cavity 7, which isdesignated the low level cavity, is provided with an appropriatemicrowave signal output connection 11, and the signal extracted at thislocation is fed to an amplifier l2 tuned to the hyperfine transitionfrequency. The output of this amplifier is used to excite cavity 8,which is designated the high level tuned cavity. This energization isaccomplished by a suitable connection 13. An output signal may be takenfrom this loop at point 14, for example.

The operation of the over-all system is as follows: Hydrogen atomsemanating from source I encounter an energy gradient withinstate-selecting magnet 2 which causes those atoms in the [F=l, m=l] and[F:l, m=l states to converge along the longitudinal axis of symmetry ofthe magnet, and those atoms in the [F :1, m=ll and [F=O, m=0] states tobe directed radially outward from this axis. Mask 4 with its centralaligned aperture 4 focuses the former atoms into the tubular extension 6of storage box 5, and this collimator further insures that only atoms inthe [F=l, m=l] and [F=l, 111:0] states reach the interior of the box.

The atoms which enter container are stored for a time interval which maylast as long as ten seconds. During this time, they may pass severaltimes through each of the two small storage bulbs 9 and and be storedtherein for a smaller interval of time. In the low level tuned cavity 7,these atoms give up sufficient energy to maintain a low level maseroscillation at the hydrogen hyperfine frequency. That is, the atomsundergo transitions to the [F=O, m=0] state, and the radiationaccompanying this transition establishes a weak, self-sustainedoscillation in cavity 7 tuned to this frequency.

The microwave signal excited from cavity 7, after amplification inamplifier 12, is coupled to cavity 8. Consequently, a relatively strongstate of oscillation at the hyperfine transition frequency is presentwithin this cavity. Atoms which enter bulb 10, therefore, are subjectedto a sufiiciently intense R.F. field at the hyperfine transition valueso that they are placed in a phase coherent, radiant state after a fewpassages through this bulb. Hence, they are in condition to contributeto the continued excitation of cavity 7 when and if they subsequentlyenter this portion of the apparatus.

Storage box 5 and the state-selection portion of the system, it will beappreciated, are evacuated by appropriate vacuum pumps, not shown. Also,the main storage box 5 and both tuned cavities 7 and 8 are surrounded byconcentric moly permalloy shields to insure a sufficiently low anduniform magnetic field within these components so that induced Zeemantransitions are not serious during the relatively extensive storagetime.

The interior wall surfaces of main storage box 5 and bulbs 9 and 10 arecoated with materials of the class described in applicantsabove-described patent for the reasons mentioned therein. The preferredcompounds as noted are the hydrogen-saturated hydrocarbons, thehydrogensaturated silicons, the silanes, and halogen-saturated compoundssuch as Teflon.

If the diameter of storage box 5 is ten times that of bulbs 9 and 10 andif the storage times in this box is ten times longer and the beamintensity ten times greater, then oscillations can be obtained in theabove system over the same fraction of range of source pressures asthose encountered in the prior art atomic hydrogen maser if theamplification provided by amplifier 12 is such as to maintain the energylevel in the high cavity 8, 60 db over that in the low cavity 7.

FIG. 2 shows an alternative arrangement in which the high level, tunedcavity 20 is relocated to a position between the defining aperture 21 ofmask 22 and the entrance 23 of the large storage container 29. Thiscavity is again energized with a signal derived from the low levelcavity mounted on a wall of storage container 29. Since the atoms in thebeam now pass through cavity 20 only once, instead of several times, theradio frequency field within it must be appreciably greater than thatwithin the corresponding high level, tuned cavity 8 of FIG. 1.Consequently, in this modification the amplifier interconnecting thehigh and low level cavities must have a greater amplification factor.Also, cavity should have appropriate aligned apertures to allow theatoms to enter this portion of the system and continue on into the mainstorage container. Because the atoms do not require prolongedinteraction with the high intensity radio frequency field within thiscavity, they may be permitted to pass directly through this devicewithout any interim storage in a dielectric bulb of the type employed inthe system of FIG. 1.

The arrangement of FIG. 2 should provide slightly greater stability ofthe output signal since the atoms are in a phase coherent radiant stateduring the entire time that they are in the large storage container 29.

It might appear that unknown phase shifts in the amplifier would lead toan erroneous output frequency. However, the usual hydrogen maserprocedure of tuning the cavity such that the frequency is independent ofthe beam intensity assures that the cavity mistuning, the phase shift inthe amplifier, and the spin exchange collisions will all be mutuallybalanced so that the maser oscillates at the true hyperfine frequency.

Obviously many modifications and variations of the present invention arepossible in the light of the above teachings. It is therefore to beunderstood that within the scope of the appended claims the inventionmay be practiced otherwise than as specifically described.

What is claimed is:

1. A maser oscillator comprising, in combination,

a storage container having an inside dimension which is large comparedto the wavelength of the radio frequency energy radiated by a gaseousatom which undergoes a magnetic dipole transition between an upper and alower hyperfine energy state;

means for directing a beam composed of such gaseous atoms into theinterior of said storage container, the predominant number of atoms insaid beam being in said upper hyperfine energy state;

a pair of cavity resonators in communication with the interior of saidstorage container;

means for extracting the radio frequency signal developed within one ofsaid cavity resonators by gaseous atoms which leave the interior of saidstorage container, enter the interior of said cavity resonator and thereundergo a magnetic dipole transition from said upper to said lowerhyperfine energy state;

means for amplifying said radio frequency signal; and

means for exciting the other cavity resonator of said pair with saidamplified radio frequency signal whereby gaseous atoms which leave theinterior of said storage container and enter the interior of this cavityresonator and which are in said lower hyperfine energy state are excitedto said upper hyperfine energy state.

2-. In an arrangement as defined in claim 1,

means accommodated within each cavity resonator for storing said gaseousatoms for an interim period of time and thereafter redirecting saidgaseous atoms back into the interior of said storage container.

3. In an arrangement as defined in claim 2, wherein said means forstoring said gaseous atoms and redirecting them back into the interiorof said storage container comprises a dielectric bulb in which cavityresonator, each dielectric bulb having an entrance passageway leadinginto the interior of said storage container and having its inner wallsurfaces made of a nonrelaxing material whereby gaseous atoms whichenter these bulbs from the interior of said storage container maycollide a multiplicity of times with said inner Wall surfaces withoutbeing disturbed appreciably from their hyperfine energy state.

4. An atomic hydrogen maser comprising, in combination,

a storage container having a size which is large compared to thewavelength of the radio frequency energy radiated by a hydrogen atomwhich undergoes a magnetic dipole transition between an upper and lowerhyperfine energy state; means for directing a beam of hydrogen atoms,the predominant number of which are in said upper hyperfine energystate, into said storage container,

said storage container having its inner wall surfaces made of anonrelaxing material which permits said hydrogen atoms to colliderepeatedly with said surfaces without experiencing any appreciablechange in their hyperfine energy state; a pair of radio frequency cavityresonators coupled to said storage container,

each of said cavity resonators being tuned to the frequency of saidradio frequency energy; a dielectric storage bulb accommodated withineach of said cavity resonators,

each of said bulbs having an entrance passageway which communicates withthe interior of said 1 storage container whereby any of the hydrogenatoms present within said storage container may enter each of saidstorage bulbs, each of said bulbs having its inner wall surfaces made ofa nonrelaxing material whereby the hydrogen atoms which enter thesestorage bulbs may collide repeatedly with these wall surfaces withoutexperiencing any appreciable change in their hyperfine energy state;means for extracting the radio frequency signal available in one of saidcavity resonators as a result of the excitation of that cavity resonatorby hydrogen atoms which enter its storage bulb and there undergoradiative transitions between said upper and lower hyperfine energystates; means for amplifying said radio frequency signal; and means forexciting the other cavity resonator with said amplified radio frequencysignal whereby those hydrogen atoms which enter its storage bulb and arein the lower hyperfine energy state as a result of having previouslyundergone a radiative transition to this state from said upper hyperfineenergy state are quickly excited to said upper hyperfine energy state.5. An atomic hydrogen maser comprising, in combination,

a storage container having a size which is large compared to thewavelength of the radio frequency energy produced by a hydrogen atomwhich undergoes a radiative transition between an upper and lowerhyperfine energy state; a first cavity resonator tuned to the frequencyof said radio frequency energy; means for directing a beam of hydrogenatoms, the

predominant number of which are in said upper hyperfine energy state,through said cavity resonator and into said storage container,

said storage container having its inner wall surfaces made of anonrelaxing material which permits said hydrogen atoms to colliderepeatedly with said surfaces without experiencing any appreciablechange in their hyperfine energy state; a second cavity resonator tunedto the frequency of said radio frequency energy and coupled to saidstorage container; a dielectric storage bulb accommodated within saidsaid second cavity resonator,

said bulb having an entrance passageway which communicates with theinterior of said storage container whereby any of the hydrogen atomspresent within said storage container may enter said storage bulb, saidstorage bulb having its inner wall surfaces made of a nonrelaxingmaterial whereby the hydrogen atoms which enter this storage bulb maycollide repeatedly with these wall surfaces without experiencing anyappreciable change in their hyperfine energy state; means for extractingthe radio frequency signal available in said second cavity resonator asa result of the excitation of that cavity resonator by hydrogen atomswhich enter its storage bulb and there undergo radiative transitionsbetween said upper and lower hyperfine energy state; means foramplifying said radio frequency signal; and means for exciting saidfirst cavity resonator with said amplified radio frequency signalwhereby those hydrogen atoms in said beam which pass through said firstcavity resonator and are in said lower hyperfine energy state encountera high intensity radio frequency field and are excited to said upperhyperfine energy state.

No references cited.

S. H. GRIMM, Assistant Examiner.

ROY LAKE, Primary Examiner.

